1. Field of the Invention
This invention relates to a method for the production of semiconductor lasers having a structure, which is effective to control a transverse mode of laser oscillation, by the use of a crystal growth technique for the formation of thin films such as molecular beam epitaxy or metal-organic chemical vapor deposition.
2. Description of the Prior Art
Recently, a single crystal growth technique for the formation of thin films such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MO-CVD), etc. has been developed which enables the formation of thin film growth layers having a thickness of as thin as approximately 10 .ANG.. The development of such a technique, although these significantly thin films have not yet been produced by liquid phase epitaxy (LPE), allowed the thin films to be applied to lasers, resulting in laser devices exhibiting new laser effects and/or superior laser characteristics. A typical example of these new laser devices is a quantum well (QW) laser, which is produced based on the fact that quantization levels are established in its active layer by reducing the thickness of the active layer from several hundred .ANG. to approximately 100 .ANG. or less and which is advantageous over conventional double heterostructure lasers in that the threshold current level is low and the temperature and transient characteristics are superior. Such a quantum well laser is described in detail in the following papers:
(1) W. T. Tsang, Applied Physics Letters, vol. 39, No. 10 pp. 786 (1981). PA0 (2) N. K. Dutta, Journal of Applied Physics, vol. 53, No. 11, pp. 7211 (1982), and PA0 (3) H. Iwamura, T. Saku, T. Ishibashi, K. Otuka, Y. Horikoshi, Electronics Letters, vol. 19, No. 5, pp. 780 (1983). PA0 (a) forming a substrate having a striped portion on its face, said striped portion being formed into a mesa, PA0 (b) forming a current blocking layer on each of said striped portion and the remaining face of said substrate, PA0 (c) eliminating said current blocking layer only on said striped portion thereby allowing electric current to flow through the substrate and form said striped portion into a terrace or a groove, as a whole, and PA0 (d) successively laminating crystal growth layers for laser operation on the whole face of the substrate in strict conformity with said terrace or groove on the face of said substrate.
As mentioned above, the single crystal growth technique, such as molecular beam epitaxy or metal-organic chemical vapor deposition, has resulted in the practical use of high quality semiconductor lasers having a new multiple-layered structure. However, the semiconductor laser is deficient in that a stabilized transverse mode of laser oscillation cannot be attained due to its multiple-layered structure.
One of the most important points requiring improvement in other conventional semiconductor lasers which are in practical use, is stabilization of the transverse mode of the laser oscillation. A contact stripe geometric laser, which was developed in the early stage of laser development, has a striped electrode to prevent electric current injected from transversely expanding, and attains a laser oscillation in a zero order mode (i.e., a fundamental transverse mode) upon exceeding the threshold current level, due to gains required for the laser oscilation are greater than losses within the active region underneath the stripe region, while the said contact stripe geometric laser produces a laser oscillation in an expanded transverse mode or a higher-order transverse mode with an increase in the injection of current beyond the threshold current level, because carriers which are injected into the active layer spread to the outside of the striped region resulting in expanding the high gain region. Due to such an unstable transverse mode and dependency of the transverse mode upon the amount of injected electric current, the linear relationship between the injected electric current and the laser output decreases. Moreover, the laser output resulting from pulse modulation is unstable so that the signal-noise ratio is reduced and its directivity becomes too unstable to be used in an optical system such as optical fibers, etc. In order to overcome the above-mentioned practical drawbacks of contact stripe geometric lasers, a variety of structures for semiconductor lasers of GaAlAs and/or InGaAsP systems have been already produced by liquid phase epitaxy, which prevent not only electric current but also a light from transversely expanding thereby attaining stabilization in the transverse mode. However, most of these semiconductor lasers can only be produced by the growth of thin film layers on a grooved substrate, a mesa substrate or a terraced substrate based on a peculiarity of the liquid phase epitaxy, typical examples of which are channel-substrate planar structure injection lasers (CSP lasers) (K. Aiki, M. Nakamura, T. Kuroda and J. Umeda, Applied Physics Letters, vol. 30, No. 12, pp. 649 (1977)), constricted double heterojunction lasers (CDH lasers) (D. Botez, Applied Physics Letters, vol. 33, pp. 872 (1978)) and terraced substrate lasers (TS lasers) (T. Sugino, M. Wada, H. Shimizu, K. Itoh, and I. Teramoto, Applied Physics Letters, vol. 34, No. 4, (1979)). All of these lasers can be only produced utilizing anisotrophy of the crystal growth rate, but not produced by the use of a crystal growth technique such as molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MO-CVD).